1. Field of the Invention
Embodiments of the invention generally relate to methods and apparatus for use in oil and gas wellbores. More particularly, the invention relates to methods and apparatus for utilizing deployment valves in wellbores.
2. Description of the Related Art
Oil and gas wells are typically initially formed by drilling a borehole in the earth to some predetermined depth adjacent a hydrocarbon-bearing formation. After the borehole is drilled to a certain depth, steel tubing or casing is typically inserted in the borehole to form a wellbore, and an annular area between the tubing and the earth is filled with cement. The tubing strengthens the borehole, and the cement helps to isolate areas of the wellbore during hydrocarbon production. Some wells include a tie-back arrangement where an inner tubing string located concentrically within an upper section of outer casing connects to a lower string of casing to provide a fluid path to the surface. Thus, the tie back creates an annular area between the inner tubing string and the outer casing that can be sealed.
Wells drilled in an “overbalanced” condition with the wellbore filled with fluid or mud preventing the inflow of hydrocarbons until the well is completed provide a safe way to operate since the overbalanced condition prevents blow outs and keeps the well controlled. Overbalanced wells may still include a blow out preventer in case of a pressure surge. Disadvantages of operating in the overbalanced condition include expense of the mud and damage to formations if the column of mud becomes so heavy that the mud enters the formations. Therefore, underbalanced or near underbalanced drilling may be employed to avoid problems of overbalanced drilling and encourage the inflow of hydrocarbons into the wellbore. In underbalanced drilling, any wellbore fluid such as nitrogen gas is at a pressure lower than the natural pressure of formation fluids. Since underbalanced well conditions can cause a blow out, underbalanced wells must be drilled through some type of pressure device such as a rotating drilling head at the surface of the well. The drilling head permits a tubular drill string to be rotated and lowered therethrough while retaining a pressure seal around the drill string.
A downhole deployment valve (DDV) located within the casing may be used to temporarily isolate a formation pressure below the DDV such that a tool string may be quickly and safely tripped into a portion of the wellbore above the DDV that is temporarily relieved to atmospheric pressure. An example of a DDV is described in U.S. Pat. No. 6,209,663, which is incorporated by reference herein in its entirety. The DDV allows the tool string to be tripped into the wellbore at a faster rate than snubbing the tool string in under pressure. Since the pressure above the DDV is relieved, the tool string can trip into the wellbore without wellbore pressure acting to push the tool string out. Further, the DDV permits insertion of a tool string into the wellbore that cannot otherwise be inserted due to the shape, diameter and/or length of the tool string.
Actuation systems for the DDV often require an expensive control line that may be difficult or impossible to land in a subsea wellhead. Alternatively, the drill string may mechanically activate the DDV. Hydraulic control lines require crush protection, present the potential for loss of hydraulic communication between the DDV and its surface control unit and can have entrapped air that prevents proper actuation. The prior actuation systems can be influenced by wellbore pressure fluxions or by friction from the drill string tripping in or out. Furthermore, the actuation system typically requires a physical tie to the surface where an operator that is subject to human error must be paid to monitor the control line pressures.
An object accidentally dropped onto the DDV that is closed during tripping of the tool string presents a potential dangerous condition. The object may be a complete bottom hole assembly (BHA), a drill pipe, a tool, etc. that free falls through the wellbore from the location where the object was dropped until hitting the DDV. Thus, the object may damage the DDV due to the weight and speed of the object upon reaching the DDV, thereby permitting the stored energy of the pressure below the DDV to bypass the DDV and either eject the dropped object from the wellbore or create a dangerous pressure increase or blow out at the surface. A failsafe operation in the event of a dropped object may be required to account for a significant amount of energy due to the large energy that can be generated by, for example, a 25,000 pound BHA falling 10,000 feet.
Increasing safety when utilizing the DDV permits an increase in the amount of formation pressure that operators can safely isolate below the DDV. Further, increased safety when utilizing the DDV may be necessary to comply with industry requirements or regulations.
Therefore, there exists a need for improved methods and apparatus for utilizing a DDV.